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 # phosphor-virtual-sensor
+
+phosphor-virtual-sensor reads in virtual_sensor_config.json
+There are two types of data in this file:
+
+#### 1) virtual sensor configuration information
+See virtual_sensor_config.json in this repository for an example. Sensors added
+this way can use any expression that is accepted by exprtk.
+
+#### 2) information to get a virtual sensor configuraton from D-Bus
+For example:
+```
+    {
+    "Desc":
+        {
+            "Config" : "D-Bus",
+            "Type" : "modifiedMedian"
+        }
+    }
+```
+Sensors added this way can only use a set of restricted calculations. At this
+stage the only type supported is modifiedMedian.
+
+The virtual sensor configuration information needs to be added into the
+relevant hardware configuration file in entity-manager. This method of adding a
+virtual sensor allows a recipe that builds for different hardware
+configurations to have different virtual sensors for each configuration.
+
+The virtual sensor configuration in entity manager follows a different format
+to the JSON in virtual_sensor_config.json (specified in
+entity-manager/schemas/VirtualSensor.json). In particular, the format for
+Thresholds is different. The following table shows the severity fields
+that correspond to a particular threshold.
+
+Threshold       | Severity
+----------------|-----------
+Warning         |     0
+Critical        |     1
+PerformanceLoss |     2
+SoftShutdown    |     3
+HardShutdown    |     4